GPS World, April 2015

INNOVATION Algorithms Methods 0 200 400 600 800 1000 1200 1400 1600 1800 50 45 40 35 30 25 20 15 10 05 00 Time seconds Ambiguity cycles FIGURE 3 Ambiguity residuals for the extra widelane 5 2 combination A Real Time PPP AR Implementation A PPP AR technique was successfully implemented by the Centre National dEtudes Spatiales CNES in real time in the so called PPP Wizard demonstrator in 2010 and has been subsequently improved In this demonstrator and in the framework of the International GNSS Service IGS Real Time Service RTS and the RTCM the GPS and GLONASS constellation orbits and clocks are computed Additional biases for GPS ambiguity resolution are computed and broadcast to the user The demonstrator also provides an opensource implementation of the method on the user side for test purposes Centimeter level positioning accuracy in real time is obtained on a routine basis Limitations of the Bias Formulations The current formulation works but it has several drawbacks The chosen representation is dependent on the implemented method Even if the nature of the biases is the same their representation may be different according to the underlying methods and this makes it dif cult for a standardization of the bias messages The user side must implement the same method as the one used on the network side Otherwise the user side would have to convert the quantities from one method to another leading to potential bugs or misinterpretations It is limited to the dual frequency case There are only two quantities to be computed in the dual frequency case and but in the triple frequency case there are many more possible combinations For example one can have this is a non exhaustive list where the indices refer to different pairs of frequencies and other ionosphere free combinations such as phase widelane only or even phase ionospherefree and geometry free combinations are possible New RTCM SSR Model The new model as proposed by the RTCM Special Committee 104 SSR working group for phase bias messages is based on the idea that the phase bias is inherent to each frequency Thus instead of making speci c combinations one phase bias per phase observable is identi ed and broadcast It is noted that this convention was adopted a long time ago for code biases Indeed in the RTCM framework and unlike the standard differential code bias DCB convention where code biases are undifferenced but combined the RTCM SSR code biases are de ned as undifferenced and uncombined The general model for uncombined code and phase biases is therefore 4 Time group delays τ and phase clocks in Equation 1 are replaced by code and phase biases and respectively RTCM SSR code and phase biases correspond to the satellite part of these biases The prime notation denotes the unbiasing process of the measurements Here the clock de nition is crucial As the biases are uncombined they are referenced to the clocks The convention chosen for the standard is natural it is the same as the one used by IGS that is in our notation GPS World April 2015 www gpsworld com 44

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